The invention relates to microwave and millimeter wave monolithic integrated circuits (MMIC), and more particularly to protection of high performance low noise receiver amplifiers from excessive rf input power, spike leakage, and electromagnetic pulses.
It is known in the prior art to provide high power limiters formed of silicon PIN diodes to protect low noise receiver amplifiers operating in the microwave to millimeter wave frequency ranges. A drawback of PIN diodes is that they are not GaAs MMIC compatible, and hence are not monolithically integratable with the receiver amplifier. This in turn requires the forming of discrete devices which are later connected in a circuit which then must be noise matched, adjusted, etc. for various resistance, capacitance, inductance and parasitic values.
The present invention overcomes the drawbacks of non-MMIC compatible limiters and provides the capability to protect low noise receiver amplifiers operating in the microwave to millimeter wave frequency ranges. The present invention adapts MOTT-barrier Schottky junctions with MMIC compatible processing to provide a limiter formed by an anti-parallel array of Schottky junctions monolithically integrated on the same substrate with the low noise receiver amplifier. The invention enables the formation of matrix junction arrays in accordance with system power protection requirements as well as monolithically integrating the limiter formed thereby with the low noise receiver amplifying elements, including MESFETs, HEMTs, HBTs, etc. The MMIC compatibility also provides the tools to reduce volume, size, weight, and cost, as well as parasitic elemental values in order to efficiently operate at very high frequencies. Furthermore, it has been found that the present approach is more effective against spike leakage and radiation tolerance than PIN diode limiters.
In one aspect of the invention, a Schottky diode array is embedded in an on-chip low pass impedance matching structure that transforms 50 ohms to Z.sub.opt, the source impedance required for the minimum FET noise figure. By absorbing the diode's junction capacitance, C.sub.jo, into the low pass structure, the bandwidth is maximized and the noise is minimized. Thus, it is possible to achieve a noise matched rf protected low noise receiver amplifier that only requires an output matching network to make it a complete low noise module. No further input matching is required. Eliminating the need for a separate noise matching network results in at least several tenths of dB reduction in input losses and a corresponding reduction in the amplifier noise figure.
The Schottky diode based high power limiter technology of the present invention provides solutions to many of the problems of PIN diode limiters, and provides superior insertion loss at microwave and millimeter wave frequencies, excellent spike and flat leakage performance, and total compatibility with MMIC fabrication techniques. Limiters in accordance with the invention having a pulsed power handling capability in excess of 100 watts, and insertion losses as much as 1 dB less than that of PIN diode limiters of comparable power handling capability, have been achieved up through 30 GHz. Spike leakage is a major problem with slow responding PIN diodes. Limiters in accordance with the invention have measured in the 1.0.times.10.sup.-9 joules range, a factor of 10 lower than an accepted safe lower limit for sub 0.5 micron gallium arsenide FETs and HEMTs.
In another aspect of the invention, high performance limiters have been fabricated using molecular beam epitaxy. This yields active layers having ultrasharp doping profiles for lowest series resistance and highest cutoff frequency. This enables production of rf protected millimeter wave transistors, including 0.25 micron gate length millimeter wave FETs having nearly 5 dB of gain at 35 GHz. The quarter micron gates can be defined by electron beam lithography.
The processes for the Schottky diodes and for the receiver amplifier, including a gallium arsenide FET, are MMIC compatible, and further accommodate integration of passive resistors, inductors, and capacitors, as well as transmission lines. In addition to the limiting action that occurs when the diode is driven to its low resistance state, the diode's junction capacitance under forward bias increases by a factor of nearly three, shifting the cutoff frequency of the matching structure to a point below the operating band, resulting in additional isolation.
As noted above, PIN diode limiter technology is subject to limitations for low noise amplifier protection. These limitations include: high insertion loss, typically 2 dB up to 18 GHz and 4 dB up to 40 GHz for pulsed power levels of 10 watts, two microsecond pulse width; flat leakage levels that often exceed 100 milliwatts, which could damage HEMT low noise amplifiers; and spike leakage levels up to 100.times.10.sup.-9 joules which would damage most MESFETs and HEMTs. Furthermore, the material and processing are totally incompatible with MMIC fabrication techniques, precluding integration of the limiter function with the low noise amplifier. In one embodiment of the present invention, pulsed levels up to 45 dBm can be handled, and still achieve an insertion loss of approximately 0.7 dB and an isolation of over 20 dB with 45 dBm applied at 16 GHz. Two microsecond pulses up to 45 dBm are safely handled by a single anti-parallel pair of diodes (1.times.1 array). A 2.times.2 array provides power handling capability of 54 dBm.